The present invention relates to a controlling apparatus of a solenoid used for driving a printing section of a printing unit (for example, a printing hammer, a wire, and the like for striking a printing element wheel), more particularly to a controlling apparatus of a solenoid for controlling electric current which flows through the solenoid in each of solenoid driving periods so as to be uniformalized even though the voltage of a drive source for driving the solenoid is unstable. This type of controlling apparatus, proposed by the same assignee in for example Japanese Provisional Publication SHO 61-263776, has been known.
FIG. 1A shows an outline of a printing mechanism of a typewriter to which the present invention is to be applied. A carriage 102 is reciprocally moved along a platen 100. The carriage 102 holds a daisy shaped printing element wheel 104 with a large number of printing elements on the peripheral position thereof. The printing element wheel 104 is connected to a printing element selection motor 100. Behind the printing element wheel 104, a printing hammer unit 108 is supported by the carriage 102. A hammer 110 of the printing hammer unit 108 is advanced and a selected printing element of the printing element wheel 104 is struck via a printing ribbon 114 from the backside of the printing element wheel 104 to a sheet 112 held by the platen 100 and thereby the desired letter is printed on the sheet 112.
The hammer 110 is driven by a solenoid 110 shown in FIG. 2. The solenoid 116 is fixed in a housing 118 of the printing hammer unit 108. Inside the housing 118, the hammer 110 is movably held by guide members 120 and 122. The hammer 110 is tensioned to the retreat position by an elastic spring 124. However, when the solenoid 116 is powered the hammer 110 advances against the tension force of the spring 124 and strikes the rear surface of the desired printing element of the printing element wheel 104 as described above. It is preferred by controlling the solenoid current to keep it constant while the hammer 110 is struck so as to make stable the impact force caused by the hammer 110. An outline of a typical control circuit for that is shown in FIG. 3.
A power supply to the solenoid 116 is turned on and off by a switching circuit 126. The solenoid current is converted into voltage value by a resister 128 for detecting the current. The resultant voltage is converted into a digital value, hereinafter named the A/D (Analog/Digital) conversion value, by an A/D converter 130 and a reference voltage generating circuit 132. The A/D conversion value is output to a controlling circuit 134. In this example, the A/D converter 130, the reference voltage generating circuit 132, and the controlling circuit 134 are included in a CPU (Central Processing Unit) 134-1. The control circuit 134 of the CPU 134-1 executes a so-called chopping control operation where it compares a digital value, i.e., the A/D conversion value, with a predetermined comparison value and turns on the switching circuit 126 when the digital value is smaller than the comparison value, while the control circuit 134 turns off the switching circuit 126 when the digital value is greater than the comparison value. Thus, the solenoid current becomes a saw shape, as shown in FIG. 4, in one printing cycle, hereinafter named a solenoid drive time period, so that the current becomes stable in accordance with the comparison value.
However, in the above solenoid control operation, since the timings of ON/OFF operations of the current flowed through the solenoid 116 depend on the processing speed of the A/D converter 130. When the processing speed of the A/D converter is low, for example, when it is incorporated in the CPU as shown in FIG. 3, the current deviation indicated as ".DELTA.I" in FIG. 4 becomes large. Especially, if the solenoid drive voltage is unstable, the variation of the current deviation ".DELTA.I" at each of solenoid driving periods becomes large, thereby the print impact force undesirably varies and then the printing quality degrades. On the other hand, when another A/D converter whose processing speed is high is used, for example, when the dedicated IC (Integrated Circuit) for the A/D converter is used, the current control circuit becomes very expensive although the above problem can be solved.